This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-085421, filed Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a photomask used in the manufacture of a semiconductor device, a photomask blank, and a method of applying a light exposure treatment to a semiconductor wafer by using said photomask.
In recent years, with progress in the degree of integration of the semiconductor device and in the miniaturization of the semiconductor element, the required pattern size is approaching the resolution limit of the light exposure apparatus. Therefore, a so-called xe2x80x9cresolution enhancement technologyxe2x80x9d such as an oblique illumination method and a phase control mask has come to be positively employed in the pattern transfer. Also, in the patterning process of a resist, the thickness of the resist film is being decreased in an attempt to extend the resolution limit and to widen the focus latitude.
In an ideal optical system, if the pattern arrangement on a mask is the same within a range affected by the optical proximity effect, the optical contrast of the transferred pattern, the exposure latitude and the profile of the resist pattern become the same.
However, with improvement in the resolution performance, a problem is actually generated that patterns are made different in the cross sectional profile of the resist film and the focusxc2x7 exposure latitude depending on the difference in the peripheral pattern (construction), even if the patterns are exactly the same in the design.
For example, a photomask having a fine pattern arranged on the entire surface is made different from a photomask having the periphery of a fine pattern covered with an opaque film in the cross sectional profile of the resist even if these photomasks are the same in the pattern design. To be more specific, in a line/space (L/S) pattern of 300 xcexcm square, in which the influence of the ordinary optical proximity effect is negligible, the light exposure sensitivity is improved so as to lower the light exposure latitude in the case where the average covering ratio of the periphery with an opaque film is 30%, compared with the case where the entire periphery is covered with an opaque film. Therefore, if a positive resist is used in the case where the average covering ratio is 30%, the head portion of the resist profile is rendered roundish.
These phenomena are considered to be caused by the irradiation of the resist with a back ground light (flare), which is not generated in an ideal optical system, so as to lower the optical contrast.
The flare in the light exposure apparatus is evaluated in general by the method described in publication 1 [J. P. Kirk, xe2x80x9cScattered light in photolithographic lensesxe2x80x9d, Proc. SPIE (1994)].
In the case of using a photomask in which a large opaque pattern, i.e., scores of micrometers square, is present on a transparent substrate, the light must not reach that region of a silicon wafer which is positioned right under the opaque pattern. However, if the exposure Dose is gradually increased, the particular region of the silicon wafer is exposed to light because of the flare. Publication 1 quoted above discloses a method of numerically expressing the amount of the flare by utilizing the phenomenon described above.
The influences given by the flare to the device pattern are described in publication 2 [C. Progler, xe2x80x9cPotential causes of across field CD variationxe2x80x9d, Proc. SPIE (1997)] and publication 3 [E. Luce, xe2x80x9cFlare impact on the intrafield CD control for sub-0.25 xcexcm patterningxe2x80x9d, Proc. PPIE (1999)].
These publications 2 and 3 teach that the amount of the flare is distributed substantially concentrically within the light exposure region. On the other hand, it has been experimentally confirmed that the size of the resist pattern is concentrically changed.
FIG. 1 schematically shows the situation of the light exposure in the case of using a conventional photomask. In the drawing, the general flare is involved in the reflection from the surface of a lens 81 (projection optics system), from the upper surface and lower surface of a photomask 82 including a quartz glass substrate 821 and a laminate film 822 consisting of a Cr film and an oxidized Cr film and acting as a opaque film, and from the surface of a wafer 83. The flare can be divided into a flare generated on the upstream side (illuminating optics system) of the photomask 82 and a flare generated on the downstream side (projection optics system) of the photomask 82. A reference numeral 841 in the drawing denotes an unexposed portion of the resist, with a reference numeral 842 denoting light-exposed portion of the resist.
In each of these cases, the amount of the flare is considered to be proportional to the covering ratio (i.e., area of the opaque film/area of the quartz glass substrate) of the photomask 82. The flare is considered to boost the light intensity distribution on the wafer 83 as a background so as to lower the contrast of the pattern.
However, as a result of an extensive research conducted by the present inventors, it has been clarified that the change in the cross sectional profile of the resist film etc., which is derived from the difference in the peripheral pattern of the photomask 82, is irrelevant to the flare as described herein later in detail.
An object of the present invention, which has been achieved in view of the situation described above, is to provide a photomask having an object pattern and a peripheral pattern formed in the periphery of the object pattern, which permits performing a light exposure that is unlikely to be affected by the peripheral pattern and to provide a photomask blank used for preparing the particular photomask.
Another object of the present invention is to provide a method of applying a light exposure treatment to a semiconductor wafer by using the photomask of the present invention.
As a result of an extensive research, the present inventors have found that the problem in respect of the change in the cross sectional profile of the resist pattern described above is caused by the light re-reflected from the back surface of the photomask. The present invention is basically featured in that the re-reflected light can be effectively suppressed.
According to a first aspect of the present invention, which has been achieved for achieving the objects described above, there is provided a first photomask, comprising a transparent substrate having a first main surface and a second main surface opposite to the first main surface, the substrate transmitting the exposed light; a pattern formed on the first main surface of the transparent substrate and having at least one of a opaque film, a translucent film, and a phase control film, the opaque film not transmitting the exposed light, the translucent film transmitting partly the exposed light, and the phase control film serving to control the phase of the exposed light; and a thin film formed on the second main surface of the transparent substrate and containing calcium fluoride.
In the first photomask, a thin film consisting of calcium fluoride is formed as an antireflection coating on the back surface of the transparent substrate. The refractive coefficient of an antireflection coating of an ideal single layer structure (reflection-reduced single layer film) is ns1/2, where ns represents the refractive coefficient of a quartz glass substrate used as the transparent substrate in the present invention.
The material having an ideal value (=ns1/2) of the refractive coefficient at KrF or ArF, which is known to the art nowadays, includes only calcium fluoride (fluorite) and a mixture of calcium fluoride and a suitable additive. Therefore, if the first photomask is used in the case where a KrF laser beam or an ArF laser beam is used as the exposed light, it is possible to prevent the re-reflection of light from the back surface of the first photomask, making it possible to carry out the light exposure treatment that is unlikely to be affected by the peripheral pattern. Incidentally, it is desirable for the thin film consisting of calcium fluoride to contain an additive effective for lowering the thermal expansion coefficient of the thin film.
According to a second aspect of the present invention, which has been achieved for achieving the objects described above, there is provided a second photomask, comprising a transparent substrate having a first main surface and a second main surface opposite to the exposed first main surface, the substrate transmitting the exposed light; a pattern formed on the first main surface of the transparent substrate and having at least one of a opaque film, a translucent film, and a phase control film, the opaque film not transmitting the exposed light, the translucent film transmitting partly the exposed light, and the phase control film serving to control the phase of the exposed light; and a film of a laminate structure formed on the second main surface of the transparent substrate and including at least a first thin film and a second thin film laminated one upon the other, the first thin film being interposed between the second main surface and the second thin film, and the refractive coefficient of the first thin film being larger than that of the second thin film.
In the second photomask, a film of a laminate structure including at least a first thin film and a second thin film is formed on the second main surface (back surface) of the transparent substrate. The refractive coefficient of the first thin film is larger than that of the second thin film.
The film of the particular laminate structure is expected to produce a large antireflection effect even if the first and second thin films are formed of the materials used for forming the ordinary antireflection coating. It follows that, if the second photomask is used, it is possible to prevent the re-reflection of the light from the back surface of the photomask so as to perform the light exposure treatment that is unlikely to be affected by the peripheral pattern.
In a third aspect of the present invention, which has been achieved for achieving the objects described above and which is directed to the second photomask described above, it is desirable for the photomask to meet the relationship of nf1xe2x89xa7nf2xc3x97ns1/2, where ns represents the refractive coefficient of the transparent substrate for the wavelength of the exposed light, nf1 represents the refractive coefficient of the first thin film, and nf2 represents the refractive coefficient of the second thin film.
Where the relationship of nf1xe2x89xa7nf2xc3x97ns1/2 is satisfied, it is possible to reduce the reflectance on the back surface of the transparent substrate to substantially zero. It follows that it is possible to carry out the light exposure treatment that is unlikely to be affected by the peripheral pattern, if the materials satisfying the relationship given above are used for forming the transparent substrate, the first thin film and the second thin film. To be more specific, it is desirable for the transparent substrate to be formed of quartz, for the first thin film to be formed of magnesium oxide, and for the second thin film to be formed of magnesium fluoride.
In the photolithography in the future, in which an ArF laser, an F2 laser, etc. are used as the light source, it is possible for the materials meeting the relationship of nf1xe2x89xa7nf2xc3x97ns1/2 not to be used or not to be present. In such a case, a high antireflection effect can be expected by using a film of a laminate structure formed of at least three layers including the first and second thin films. However, the dependency of the reflectance on the incident angle tends to be narrowed with increase in the number of thin films laminated one upon the other no matter what materials may be combined for forming the thin films. Therefore, the upper limit in the practical number of layers included in the laminate structure is about 10.
An antireflection coating is known to the art for a long time. The conventional antireflection coating is intended to prevent the generation of the flare caused by the reflection from the interface between the transparent substrate and the pattern formed of a opaque film and formed on the surface of the transparent substrate and to prevent the generation of the flare caused by the reflection from the surface of the pattern, because the reflections pointed out above occupy substantially all the reflection from the entire photomask. However, the flare that is to be suppressed in the present invention is not the flare caused by the conventional reflection but the flare caused by the reflection from the back surface of the transparent substrate. It follows that the present invention quite differs from the prior art in the flare that is to be suppressed.
FIG. 2 shows the situation of the light exposure treatment in the case of using the photomask of the present invention. Unlike the conventional photomask 82 shown in FIG. 1, an antireflection coating 85 is formed on the back surface of a quartz glass substrate 821 in the photomask of the present invention as shown in FIG. 2.
The photomask of the present invention is featured in that the antireflection coating 85 is formed of a material having an optical constant effective for lowering the reflectance of the reflected light of the exposed light that is incident on the interface between the quartz glass substrate 821 and the light exposure atmosphere (the air atmosphere), and that the antireflection coating 85 is formed in a thickness effective for lowering the reflectance of the reflected light noted above. It is desirable for the material of the antireflection coating 85 not to absorb (not to attenuate) the wavelength of the exposed light or to be very small in the absorption (attenuation). Where the material performs the absorption, it is desirable for the material of the antireflection coating 85 not to be changed by the crystallization or the generation of the point defect or the like.
In the case of using the photomask of the present invention, the reflected light on the upper surface of a wafer 83 passes through a lens 81 (projection optical system) having an antireflection measure applied thereto substantially completely so as to arrive at the surface of the photomask 82. The most of the reflected light on the surface of the wafer 83 is brought back to the position on the photomask 82 corresponding to the position on the wafer 83. In this case, since the antireflection coating 85 permits suppressing the reflected light (re-reflected light) at the interface between the back surface of the photomask 82 and the light exposure atmosphere (the air atmosphere), it is possible to effectively prevent the generation of the flare caused by the reflection from the interface noted above. It follows that it is possible to suppress sufficiently the reduction of the exposure latitude during use of the photomask having a high opening ratio. As a result, even in the case of using a positive resist and the photomask having a high opening ratio, it is possible to obtain a resist pattern 87 having a rectangular profile at head portion, which is satisfactory, as shown in FIG. 3B, as in the case of using a positive resist and the photomask having a low opening ratio, not a resist pattern 86 having a roundish head portion as shown in FIG. 3A.
According to a fourth aspect of the present invention, which is intended to achieve the objects noted above, there is provided a photomask blank, comprising a transparent substrate having a first main surface and a second main surface opposite to the first main surface, the transparent substrate transmitting the exposed light; a thin film formed on the first main surface of the transparent substrate and including at least one of a opaque film, a translucent film and a phase control film, the opaque film not transmitting the exposed light, the translucent film partly transmitting the exposed light, and the phase control film serving to control the phase of the exposed light; and a film of a laminate structure formed on the second main surface of the transparent substrate and including at least a first thin film and a second thin film laminated one upon the other, the first thin film being interposed between the second main surface of the transparent substrate and the second thin film, and the refractive coefficient of the first thin film being larger than the refractive coefficient of the second thin film.
According to a fifth aspect of the present invention, which is intended to achieve the objects noted above, there is provided a method of applying a light exposure to a semiconductor wafer, comprising a first step of calculating design data on the basis of the device function; a second step of preparing a photomask for acquisition of the optical proximity effect correction (OPC) rule on the basis of the design date; a third step of performing a light exposure for patterning a thin film on the wafer by using the photomask for acquisition of the OPC rule; a fourth step of measuring the size of the pattern formed by the light exposure on the wafer; a fifth step of acquiring the OPC rule on the basis of the size of the pattern obtained by the measurement; a six step of correcting a pattern size on a photomask by using the acquired OPC rule such that a pattern satisfying the design data in the first step is obtained on the wafer; a seventh step of preparing the photomask having the pattern size, the photomask comprising a transparent substrate having a first main surface and a second main surface opposite to the first main surface, the substrate transmitting the exposed light; a pattern to be transferred formed on the first main surface of the transparent substrate; and an antireflection coating formed on the second main surface of the transparent substrate; and an eighth step of performing a light exposure for patterning a thin film on the wafer by using the photomask prepared in the seventh step.
In the method of applying a light exposure treatment to a semiconductor wafer, which is constructed as described above, it is possible to shorten the acquisition time of the optical proximity effect correction (OPC) rule in the step of applying a light exposure treatment to the semiconductor wafer and to improve the dimensional accuracy of the pattern formed by the light exposure step by using the first photomask as the photomask for acquiring the OPC rule and by using the first photomask in the step of applying the light exposure treatment to the semiconductor wafer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.